Lump coal must be pulverized prior to use as fuel in a combustion chamber of the type frequently used to make steam in electrical power generating plants. The coal is typically pulverized using a vertical air-swept pulverizer comprising a milling bowl onto which lump coal is introduced for grinding and pulverizing by one or more large grinder or crusher wheels. Air is forced to flow upwardly through the interior of the pulverizer housing toward a classifier structure mounted near the top of the pulverizer. The primary function of the classifier is to segregate the airborne particles according to size whereby finer particles exit the pulverizer while larger particles are returned to the milling bowl for further size reduction.
It is well understood that particle size or “fineness” is an important factor in the satisfactory operation of a boiler. In general, the finer the particle size, the greater the ratio of coal particle surface area to overall fuel weight and the more efficient the combustion process. Coal particles greater in size than 300 μm are the largest contributors to unburned carbon residues and fly ash and in-chamber corrosion.
The prior art classifier is a generally cylindrical structure mounted near the top of the pulverizer housing. The larger coal particles are directed by the classifier into a cone immediately under the classifier. The classifier has circumferential intake ports of fixed size and a series of vanes inside of the ports to impart spin to the incoming airborne coal stream. In most classifiers, intake characteristics are varied by individually adjusting the vanes to different angles, a time-consuming and laborious process. In other classifiers, the vanes are interconnected by a complex linkage so they can all be adjusted as to angle in one operation. The linkage is subject to clogging and jamming and requires regular maintenance to remain operational.